A number of vanadium catalysts have been described in the literature based on species such as VCl.sub.4, VCl.sub.3, VOCl.sub.3, and vanadyl tris-acetylacetonate. Impreqnated on or chemically anchored to supports such as silica, these catalysts provide ethylene copolymers with a broad molecular weight distribution and certain desirable properties. However, these catalyst systems are plagued by several problems, among them, low activity. Recently vanadium catalysts have come to the fore in ethylene copolymer production because of the discovery of a particular catalyst composition, which is singularly effective in gas phase processes. The advantages of this catalyst composition are superior hydrogen response thus achieving very low levels of unsaturation; a high degree of productivity resulting in very low catalyst residues; and the production of ethylene copolymers having a relatively broad molecular weight distribution and a wide melt index range. Further, it permits (i) the utilization of a broad range of comonomers, which enables the production of a wide range of densities; (ii) a high level of incorporation of comonomers into the copolymer structure; and (iii) a more random distribution of the comonomers in that structure. It also provides polymers having high bulk densities and readily fluidizable particle sizes low in fines and substantially free of agglomeration. The particular catalyst composition referred to comprises (i) the reaction product of a vanadium trihalide and an electron donor, which together with a modifier are impregnated onto a silica support; (ii) a halocarbon promoter; and (iii) a hydrocarbyl aluminum cocatalyst.
A major problem with this composition arises, however, in connection with the manufacture of copolymers from more than two comonomers. For example, under the low temperature polymerization conditions used in the preparation of ethylene/propylene/ethylidene norbornene terpolymer rubber, the third comonomer, ethylidene norbornene, severely reduces the activity of this usually effective catalyst.